1. Field of the Invention
The present invention relates to an optical recording medium having an optical recording layer for optically recording information and a method of producing the same.
2. Description of the Related Art
In recent years, in the field of information recording, research has been underway in various locations on optical information recording systems. Optical information recording systems have a number of advantages such as a capability of non-contact recording and reproduction, a capability of handling different forms of memories such as read only, write-once, and rewritable types. Broad applications from industrial use to consumer use can be envisioned for these as systems enabling realization of low cost, large size files.
A larger capacity of the optical recording media for the above various types of optical information recording systems (hereinafter also referred to as an “optical disk”) has been achieved mainly by shortening the wavelength of the laser light serving as the light source used in the optical information recording systems and using lenses having larger numerical apertures to reduce the spot size on the focal plane.
For example, in a CD (compact disk), the laser light wavelength is 780 nm, the numerical aperture (NA) of the lens is 0.45, and a capacity is. 650 MB, while in a DVD-ROM (digital versatile disk—read only memory), the laser light wavelength is 650 nm, the NA is 0.6, and the capacity is 4.7 GB.
Furthermore, in the next generation optical disk systems, a larger capacity of 22 GB or more can be achieved by using an optical disk including an optical recording layer over which is formed a thin light transmitting protective film (cover layer) having a thickness of, for example, about 100 μm and making the laser light wavelength 450 nm or less and the NA 0.78 or more.
FIG. 1 is a schematic sectional view of the structure of an optical disk for the above next generation optical disk systems.
For example, an optical recording layer 11 comprised of an aluminum or other metal film or a phase change type thin film made of inorganic material or other and having irregularities of lands L and grooves G is formed on a substrate 10 comprised of a polycarbonate and having a thickness of about 1.1 mm.
The optical recording layer 11 is coated by, for example, an ultraviolet curing resin to form a light transmitting protective film 14.
The above optical disk is a type of optical disk where laser light is focused on the optical recording layer 11 through the protective film 14 and the reflected light is read. By making it above type, a higher numerical aperture can be achieved.
The method of producing the above conventional optical disk having the above optical recording layer will be explained next.
First, a pattern of irregularities for the optical recording layer is formed by for example injection-molding on a substrate comprised for example of a polycarbonate and having a thickness of 1.1 mm, then aluminum etc. is deposited thereon by for example sputtering so as to form a reflection film having a pattern corresponding to the above pattern of irregularities, that is, the optical recording layer 11.
Next, a liquid-state ultraviolet curing resin is supplied by spinning coating on the optical recording layer 11 from a dispenser while making the substrate 10 spin so as to form an ultraviolet curing resin film.
Next, ultraviolet light is irradiated on the ultraviolet curing resin film to cause the same to cure and form a protective film. By this, an optical disk having the structure shown in FIG. 1 can be obtained.
However, in the above process of forming an ultraviolet curing resin film, there was the problem that the ultraviolet curing resin film for forming a protective film could not be formed uniformly on the substrate due to the high viscosity of the ultraviolet curing resin.
FIG. 2 is a schematic sectional view of the state in the step of forming the above ultraviolet curing resin film.
A not shown optical recording layer is formed on the substrate 10. When coating an uncured film 14a of an ultraviolet curing resin thereon by spin coating, since the viscosity of the ultraviolet curing resin is high, the thickness of the uncured film 14a ends up becoming vastly different between near the center of the substrate (region expressed by x in the figure) and near the periphery (region expressed by y in the figure). It was thus difficult to obtain a uniform thickness.
Also, as explained above, at the time of curing the ultraviolet curing resin film by irradiating ultraviolet light, the cure shrinkage of the resin is large, so there was the problem that the optical disk produced ended up warping by a large degree. Also, a change in temperature or humidity also became a cause of warping because of the difference in the coefficient of expansion and water absorption of the protective film.
The amount of spherical aberration caused when the thickness of the above protective film deviates from its set value is proportional to NA4/λ for a laser light wavelength λ and a numerical aperture NA, so when shortening the wavelength of the laser light and raising the numerical aperture to increase the capacity as explained above, the characteristics required in the optical disk for keeping down spherical aberration become stricter and the uniformity of the thickness of the protective film has to be improved.
Also, the amount of coma aberration occurring when warping (tangential or radial skew) of a disk occurs is proportional to NA3/λ, so in the same way as above, when shortening the wavelength of the laser light and increasing the numerical aperture to increase the capacity, the characteristics required in the optical disk for keeping down coma aberration become stricter and warping of the disk has to be suppressed: